Metallurgical Thermodynamics And Kinetics Of Oxygen And Nitrogen In Nickel-based Alloys

Oxygen(O)and nitrogen(N)are important gas impurity elements in nickel-based superalloys.They mainly exist in the form of non-metallic inclusions such as oxides or nitrides,which have an important impact on the mechanical properties of the alloys.The main way to eliminate the above influence is to reduce the oxygen and nitrogen content in the melt by the purification smelting method.The formulation of a suitable metallurgical process requires metallurgical thermodynamics and kinetics as a guide.Therefore,it is necessary to analyze the thermodynamic limits of melt denitrification and deoxidation from the perspective of thermodynamics,and the limiting steps and reaction order of the denitrification reaction from the perspective of kinetic.These will help to clarify the metallurgical properties of denitrification and deoxygenation in liquid nickel-based alloys,thereby simplifying the process flow and reducing costs.By means of sampling methods,we have determined the solubility of nitrogen in liquid pure Ni,Ni-V,Ni-Nb,and Ni-Ta binary alloys,Ni-Cr-V,Ni-Cr-Nb,Ni-Fe-Nb,and Ni-Cr-Ta ternary alloys,Ni-19Cr-23Fe-5Nb and Ni-9Cr-10Co-3Mo quaternary alloys within the temperature range from 1773 K to 1873 K.It has been found that Sieverts' formalism successfully describes the solubility of nitrogen in all above liquid alloys under the experimental nitrogen partial pressure.The effect of temperatures on the solubility of nitrogen in pure nickel melt is determined aslog[w(N)/w(?)]Ni=-2566/T-1.64 under nitrogen partial pressure p(N2)=101 kPa in the temperature range from 1773 to 1923 K.Moreover,the addition of vanadium,niobium,and tantalum increases nitrogen solubility,particularly vanadium.Meanwhile,the activity coefficient of nitrogen,interaction coefficients,enthalpies,and entropies of nitrogen dissolution in the above liquid alloys have been determined.For example,the first-order interaction coefficients are determined as:eN(Ni)V=30.0/T-0.1783,eN(Ni)Nb=-26.6/T-0.0643,and eN(Ni)Ta=66.7/T-0.0952,respectively.In addition,the second-order cross-interaction coefficients between nitrogen and chromium or iron with niobium,and nitrogen and chromium with vanadium(or tantalum,cobalt)at 1873 K have been determined as rNCr,Nb=+0.0256,rNFe,Nb=+0.0115,rNCr,V=-0.0017,rNCr,Ta=-0.0022,and rNCr,Mo=-0.0098,respectively.Finally,based on the interaction coefficient proposed by Wagner,Lupis,and Elliott,a theoretical model and a database for predicting the solubility of nitrogen in multi-component nickel-based alloys have been improved.In the binary Ni-j systems(j=Ti,V,Cr,Nb,Ta,Mo,W,Fe,Co),the sequence of the increasing effects on the nitrogen solubility can be specified as Ti>V>Cr>Nb>Ta>Mo>W>Fe>Co>Al.The equilibrium of carbon and oxygen in molten Ni,binary alloys Ni-j(j=Fe,Co,Cr,Mo,W,V,Nb,and Ta)has been measured by equilibrating the alloy melts with continuously flowing CO-CO2 mixtures.As the result of this experiment,the Gibbs free energy of the reaction[C]+[O]=CO(g)in pure nickel melt is determined as?GC-O(Ni)(?)=-55.5-4.59×10-3 T kJ/mol in the temperature range from 1773 to 1873 K,and the value of eOC at 1873 K is determined as-0.552.The effects of the alloying elements on the activity coefficient of carbon and oxygen in molten nickel have been experimentally determined,and the first-order interaction coefficients eCj and eOj(j=Fe,Co,V,Nb,Ta,Cr,Mo,and W)are established at 1873 K,respectively.Furthermore,it is found that the carbon-oxygen product increases with the addition of V,Cr,Nb,and Fe,and the sequence of the effects on the equilibrium of carbon and oxygen can be specified as V>Cr>Nb>Fe>Mo>W>Ta>Co.Ultimately,a theoretical model and a database for predicting the oxygen content in the multi-element nickel-based alloy melt are improved under a specific carbon monoxide partial pressure and carbon content.The solubility of carbon in pure nickel melt and binary alloys Ni-j(j=Fe,Co,Cr,Mo,W,V,Nb,and Ta)have been investigated by using graphite crucibles equilibrating with the alloy melt.The temperature dependence of graphite solubility in nickel was determined as log NC=-531.9/T-0.690 in the temperature range from 1773 K to 1923 K.Meanwhile,the solubility of carbon in molten binary Ni-j(j=Co,V,Mo,W,Nb,and Ta)alloys is found to be a positive correlation to the alloying content.When the iron content is less than about 20 at.%,the solubility of carbon decreases with the increase of iron content.The first-order interaction coefficients ?Cj for j=Fe,Co,Cr,Mo,W,V,Nb,and Ta at 1873 K are also determined as+1.07,-0.27,-2.17,-2.63,-2.55,-1.63,-3.16,and-3.23,respectively,which are significantly different from that of dilute solution.The kinetics of nitrogen removal from the molten IN718 class quaternary alloy has been investigated by the sampling method under reduced pressure.The effects of initial nitrogen concentrations and alloying elements(carbon,titanium,and aluminum)on the nitrogen removal are discussed in the temperature range from 1773 K to 1923 K.Experimental results indicate that the nitrogen removal is a 1.5th-order reaction at different initial nitrogen concentrations and temperatures,and the nitrogen removal reaction is controlled by the mixed control model,including first-and second-order reactions.Moreover,the nitrogen removal reaction can be accelerated when the[C]+[O]=CO(g)reaction exists in the melt after the addition of carbon,and the kinetics changes gradually from a 1.5th-order reaction to a second-order reaction with increasing temperature from 1773 K to 1873 K,indicating that the C-O reaction significantly improves the mass transfer process of nitrogen in the melt,and the interfacial reaction becomes a limiting step.It reveals that the rate constant of the nitrogen removal increases with increasing temperature.With either the addition of titanium or the addition of both titanium and aluminum,the nitrogen removal is still a 1.5th-order reaction.It is also found that titanium decreases the rate constant of nitrogen removal,while the addition of both titanium and aluminum increases the rate constant of nitrogen removal,indicating aluminum significantly increases the rate constant of nitrogen removal.